The present invention relates to a sound generating apparatus with a sounding body driven by a piezoelectric element, which is well adaptable for an alarm sounding device for use in an automobile, for example.
In the case of an alarm sounding device, such as a horn mounted to an automobile, 100 dB or more of sound pressure is required at a position 2 m from the horn. If the horn is constructed using a diaphragm driven by a piezoelectric element, the diameter of the diaphragm must be 90 mm or more. However, in constructing the piezoelectric element for driving the diaphragm, there is a limit to which the piezoelectric element can be increased. The maximum size permitted is 50 mm.
In constructing an alarm sounding device for an automobile by using a sounding body in which a piezoelectric element is attached to the diaphragm, using the second-order resonance of the diaphragm driven by a piezoelectric element has been proposed. In FIG. 1, for example, there is shown an arrangement of a conventional sound generating apparatus based on such a second-order resonance. A sounding plate 11 consists of a diaphragm 12 laminated with a piezoelectric element 13 shaped like a thin plate. The sounding plate 11 is fit to a first housing 14 to close an opening of the first housing 14. A second housing 15 is further fit to the opening of the first housing 14 to firmly hold the sounding plate 11 between the first and second housings 14 and 15. A number of sound passing holes 161, 162, . . . , are formed in the major surface of the second housing 15. The second housing 15 also contains an air layer 17 confined therein. Vibration of the sounding plate 11 acts on the air layer 17 to generate a sound. The sound generated is radiated to the exterior through the sound passing holes 161, 162, . . . .
The first housing 14 is provided at the bottom adjacent a sound drive circuit 18. A sound drive signal is supplied from the sound drive circuit 18 to the sounding plate 11 through a pair of lead wires 19 and 20.
FIG. 2 shows a configuration of the sound drive circuit 18. An oscillating circuit 21 operates as a signal source and oscillates to produce a signal, which in turn is amplified by an amplifier circuit 22. The amplified signal is boosted by a boosting transformer 23 and then drives a sounding device 24 made up of the sounding plate 11.
In designing the sound generating apparatus thus arranged, particularly in designing the sound resonance, the second resonance frequency f.sub.p of the sounding plate 11, the diameter 2a of each sound passing hole 161, 162, . . . of the second housing 15, the number n of the holes, the length l of the hole, and a volume V of the air layer 17 are appropriately selected using known formulae. For example, the length l of the hole is determined by the thickness (2 mm) of the second housing 15. In order to obtain a satisfactorily large sound pressure, it is necessary to select relatively large areas for each hole 161, 162, . . . to obtain a satisfactory amount of the volume V.
In the example shown in FIG. 1, for tuning a sound frequency f.sub.p at 1550 Hz, the diameter 2a of each hole is 4.8 mm, the number of holes is 24, the volume V is 90 cc, and the second housing's depth h=15 mm. In this case, the amplifying effect is approximately 8 dB.
In the sound generating apparatus thus arranged, the frequency response is configured such that the smaller the low frequency sound pressure, which becomes the fundamental frequency, the smaller the amplifying effect. Therefore, the second-order resonance characteristic mainly contributing to the sound pressure is too sharp. The result is that the sound generated is loud, noisy, and high-pitched. Thus, the sound generating apparatus cannot generate a gentle or soft sound. In this respect, the sound generating apparatus provides a poor tone.
As a means for widening the width of the peak of the resonance, Utility Model Disclosure No. 58-40717 proposes an arrangement in which two diaphragms with different frequencies are arrayed in parallel. Such an arrangement, however, has no means to cope with phenomena peculiar to an acoustic oscillation of low frequencies and also no means which effectively amplifies the sound generated from a couple of sounding plates. For this reason, it was very difficult to obtain a sounding characteristic satisfactory for the alarm sounding device of the automobile with the prior art sound generating apparatus.